1. Field of the Invention
The present invention relates to an image forming apparatus that has first and second image carriers configured by positively-charged single layer type electrophotographic photoreceptors.
2. Description of the Related Art
An electrophotographic image forming apparatus, such as a copy machine, a printer, a fax machine, or a complex machine with the functions of the abovementioned devices, has, for example, a photosensitive drum functioning as an image carrier, a charging device for uniformly charging a circumferential surface of the photosensitive drum, an exposure device for forming an electrostatic latent image based on image data onto the photosensitive drum, a developing device for developing the electrostatic latent image on the photosensitive drum into a toner image, and a transfer device for transferring the toner image on the photosensitive drum to a recording medium, such as a sheet of paper, via an intermediate transfer belt and the like.
Examples of the charging device of such an image forming apparatus include a contact charging type charging device and a non-contact charging type charging device. The contact charging type charging device is known to be capable of preventing generation of ozone, unlike the non-contact charging type charging device.
Some contact charging type charging devices have charging rollers. Some conventional devices have charging rollers of electrophotographic apparatuses that use two-component toners, the charging rollers each having a shaft body, a base rubber layer formed on an outer circumference of the shaft body, and a surface layer that is formed on an outer circumference of the base rubber layer directly or with a layer interposed therebetween. In such charging rollers, the base rubber layer is made of a base rubber layer forming material that consists mainly of a rubber component having a JIS-A hardness of 15° or lower, and the surface layer is made of a surface layer forming material that has an elongation (Eb) according to JIS K6251 of 5 to 90% and a tensile strength (TS) of 35 MPa or higher.
Examples of the image carrier used in the image forming apparatus described above include an inorganic photoreceptor that has a photosensitive layer made of selenium or other inorganic material, and an organic photoreceptor that has a photosensitive layer consisting mainly of organic components such as a binder resin, a charge generating agent and a charge transport agent. Examples of the organic photoreceptor include a single layer type organic photoreceptor that has a photosensitive layer containing a charge generating agent and charge transport agent together. A single layer type organic photoreceptor that is charged positively is called “positively-charged single layer type electrophotographic photoreceptor.”
The organic photoreceptors such as positively-charged single layer type photoreceptors are likely to have lower durability than the inorganic photoreceptors. In addition, compared to the non-contact charging type charging device, the contact charging type charging device tends to impose a larger load on its photoreceptor.
Although the conventional device described above is capable of obtaining copied or printed images of high quality over a long period of time, using the charging roller of the conventional device as a charging roller of a charging device for charging such a positively-charged single layer type electrophotographic photoreceptor is not enough to form images of sufficiently high quality. Especially immediately after rotating the photosensitive drum for several tens of seconds without charging the photoreceptor, the charging performance of the photoreceptor drops significantly, and consequently fogging occurs if the image formation is continued.
An image forming apparatus with a plurality of photosensitive drums, such as a full-color image forming apparatus, tends to rotate the photoreceptors without charging the photoreceptors. The full-color image forming apparatus generally adopts a so-called tandem configuration in which black, cyan, magenta and yellow image forming units arranged therein form toner images, which are then primarily transferred to an intermediate transfer belt and then secondarily transferred to a sheet. These types of tandem image forming apparatuses include an inexpensive apparatus that is not equipped with a primary transfer roller release mechanism. In such an apparatus, color image photosensitive drums are rotated when forming a black-and-white image. In this case, the color image photoreceptors are not charged, which lowers the charging performance of the photoreceptors. This type of image forming apparatus cannot form images quickly because the photoreceptors need to be repeatedly charged and destaticized for several minutes in order to recover the charging performance of the photoreceptors.